Function of the B+ charging line for trailer batteries

Hi,

According to the pinout diagram, pin 10 of the 13-pin trailer connector is the B+ charging line for a trailer battery, and pin 11 is the corresponding ground line.
The purpose of this circuit is likely to charge a battery, such as one installed in a caravan or campervan.

Here are a few questions I have about that:

1. How is the official wiring done? Simply connecting pin 10 to the positive terminal and pin 11 to the negative terminal of the trailer battery seems too simple and unreliable to me. There should be at least one fuse between the trailer battery and the connector, and ideally also a type of switch relay, so that the trailer connector doesn't have a constant 12V supply, which could lead to leakage current and corrosion in humid conditions.

'What is the typical current capacity of such a charging circuit?' If the trailer battery is discharged and the alternator of the towing vehicle is supplying 14V, then quite significant charging currents can accumulate, which are 'only' limited by the internal resistance of the charging cable.

'Does anyone know if the wiring for the trailer hitch is pre-wired from the factory on a VW Touran, if the vehicle is equipped with a tow hitch from the factory?' And, if so, what is the cross-sectional area and load-bearing capacity/safety factor?

4. Is pin 11 (battery charging circuit ground) also connected, or does it 'only' serve the purpose of preventing the charging current from flowing through the ball joint and its undefined resistance?

I'm sure there are some campers among you who have already done a lot of research on this topic?
I would therefore be grateful for any information on this topic. A link to a thread would also be welcome, in case the topic has been discussed before. Unfortunately, the search did not yield the desired results.

Thank you and best regards.

Eike.

Sure, here's the translation:

'Addendum:'

There's also pin 9 (permanent positive / terminal 30). To activate the trailer connector, you could connect a relay to pin 9 and use it to switch the charging line to the trailer battery, so that it is activated as soon as the trailer is plugged into the socket on the coupling. The question is, whether the constant power supply is wired directly to the can.
What is the typical load capacity and level of protection offered by the 'Dauerplus' connection in the AHK (trailer hitch) socket?

Greetings.

Eike.

Hi,

After it stopped making noise, I crawled halfway under the car and took a closer look at the Touran's power outlet.
Pins 10, 11, and 12 are not connected; there are no visible contacts from the outside.
Thus, the questions regarding the battery charging circuit have been resolved for the time being.

Pin 9 (constant power) is present and also provides voltage.
The question now is which fuse is responsible for that circuit, and how much power I'm allowed to draw from it.
Does anyone have any information about that? This thread doesn't have to turn into a monologue...

Greetings & thank you for reading.

Eike.

There should be a 30 or 40A fuse. This provides continuous voltage to the rear of the vehicle via a 4mm² cable. This also powers the (if present) sockets in the trunk.

Unfortunately, there can be significant variations within the different platforms and model series. I've even noticed that there was extra wiring installed.

Hi,

'While a 30-40A fuse can certainly protect a 4mm² cable in the event of a short circuit, it's unlikely that a 4mm² cable would be used for a standard wall outlet.' From where is it distributed further within the rear of the vehicle?
In my Touran, all the power sockets are only energized when the ignition is on. That would suggest that the continuous power line has its own fuse.
I think I'll ask my mechanic about it. Let's see what they know. They should actually have circuit diagrams for those as well.
If absolutely necessary, you could try a 'brute force' method: simply short-circuit the terminals on the device and observe which fuse trips. The connection is probably not secured.

Greetings.

Eike.

I recently installed a tow hitch on my Passat 3C, and as a caravanning enthusiast, I've also delved quite deeply into the wiring involved.

VAG generally only provides a constant positive connection (Pin 9). The charging line is not connected in any of the wiring diagrams I have examined (Passat, Phaeton, A6, A. And you're writing it like that specifically for the Touran.
In the circuit diagrams I reviewed, the wiring was always specified as 2.5mm² and protected with a 20A fuse. (Of course, in the caravan, there is also a fuse directly connected to the battery).

Generally, the ground wire is never switched. The reason for the individual ground wires (pin 11 and 13) is the voltage drop. While driving, a 12V refrigerator in the caravan typically consumes around 10 amps. Over the cable length from the vehicle battery to the caravan battery (approximately 10 meters), a voltage drop of 0.5 - 1V occurs. This applies to both the ground wire and the 12V wire. Consequently, the caravan battery is no longer effectively charged. Therefore, separate ground and 12V lines are used, so that the current drawn by the refrigerator and the resulting voltage drop do not affect the charging line.

Therefore, the charging cable should always be disconnected from the battery directly and not simply connected to the mains socket via a relay from the constant power line. Of course, this is only relevant if you actually want to use a caravan with its own battery and also need to charge it over longer distances.

High-end electric vehicles typically have step-up converters before the battery, which increase the voltage of the charging line to 13.8 - 14.2V for the battery. However, this feature is only standard equipment on a very small number of WoWas.

By the way, in my Passat's owner's manual, it already specifies which fuses are used for the trailer hitch .

A 20A fuse is sufficient, even if the auxiliary battery is empty. An empty battery has approximately 12V, while the alternator provides around 14.4V. The voltage drop on the ground and B+ lines automatically limits the charging current, preventing the fuse from blowing.

My tip, if you really want to pull power (and this is so new that the corresponding pins are even present in the plug!):
- Connect the ground wire from pin 11 to a vehicle ground point in the rear.
- Route pin 10 separately, through a fuse and relay, directly to the battery or, at the very least, to the fuse box.
(Both with 2.5mm²)

If you have any further questions, please don't hesitate to ask.

PS: If your signature had a model year, I could have provided you with more specific values .
PS2: This should be either SC40 or SC41, depending on the model year. It is always 20A.


Regards,
Guste.